The core network of a mobile communication network is evolving towards IMS of All IP network. However, such the evolution cannot be completed in a short time, and replacement of the current circuit domain network with the IMS will be completed step by step, and this will be a long process. During this process, the coexistence of the IMS and the circuit domain network cannot be avoided.
As for network coverage, some areas may be covered by a single Circuit Switch(CS) network, some areas may be covered by a single IMS network, while some areas may be covered by both of the circuit switch network and the IMS network. With the evolution of the network, the coverage area of the IMS will become broader and broader, and the circuit domain will disappear gradually.
The voice call continuity (VCC) technology is proposed based on the above background. The start point of this technology lies in that an user subscribes both networks and a terminal is a double-mode terminal. When the user initiates a voice session process in one network, the signals of the network decrease, while the other network has fine signals, switching of the networks will occur, for example, the user is switched from the IMS network to the circuit domain network. In this case, the VCC technology is needed to ensure the voice session process will not be interrupted, i.e. ensure the continuity of the session.
The support of an anchoring technology is required in holding the continuity of a voice session process during switching networks. Said anchoring refers to inserting a control point at a session path during a call, and as shown in FIG. 1, the session between the UE (User Equipment) A and the UE B becomes two sessions, i.e. the session between A and the anchoring point C, and the session between the anchoring point C and B, respectively. In this case, if network switching occurs at the terminal A, the session between A and the anchoring point C will be re-established, but the session between the anchoring point C and the terminal B will not be affected or disconnected. In this way, the session between A and B can continue after re-establishing the session between A and the anchoring point C, thereby ensuring the voice session continuity.
In a VCC service, an user subscribes both networks, and each network can allocate a number to the user, for example, the circuit switch network allocates a number of E.164, and the IMS network allocates a SIP URI.
In the prior art, when a call from the circuit domain network reaches the home circuit domain network of the VCC user (the called party number is the one allocated by the circuit switch network), and a method based on an intelligent network or number portability is adopted during an anchoring process. The features of the anchoring process are shown in FIGS. 2 and 3, and for purpose of simplicity, only the process relevant to the background of the present invention will be described hereafter.
FIG. 2 is a call flow of scheme for call termination anchoring in CS domain of an intelligent network, comprising the following steps:
Step 100, the call message ISUP: IAM message of circuit switch network reaches the MSC (Mobile Switching Center) (in general, this MSC is a gateway MSC) of the home circuit domain CS network of a VCC user, and the call message includes a calling party number (CgPN) and a called party number (CdPN);
Step 110, the MSC sends a location query request to a HLR (Home Location Register);
Step 120, the HLR sends a location query request response to the MSC, wherein a trigger list includes the address of a wireless intelligent network signaling control point WIN SCP;
Step 130, the MSC sends an ANLYZD message which includes the called party number to the WIN SCP;
Step 140, the WIN SCP forwards the ANLYZD message to a VCC AS (Voice Call Continuity Application Server) after receiving the message;
Step 150, the VCC application server stores the called party number and established an association between the called party number and a subscriber after receiving the ANLYZD message. A returned anlyzd response message includes a temporary IMS routing number associated with the VCC application server.
Step 160, the WIN SCP forwards the anlyzd message to the MSC;
Step 170, the MSC sends an ISUP call message to the Media Gateway Control Function (MGCF) of the user home network according to the routing information in the anlyzd message;
Step 180, the MGCF initiates a call request (SIP Invite request) to the VCC application server after receiving the call, and this call is completed via an I-CSCF (Interrogating Call Session Control Function); and
Step 190, the VCC application server finds the called party number stored in the Step 150 according to the temporary IMS routing number associated with the VCC application server after receiving a call request, and the VCC AS uses a real called party number to continue the call.
FIG. 3 is a call flow of call termination anchoring of CS domain in the number portability scheme, comprising:
Step 210, circuit domain queries number portability database.
Step 220, a response returned by the number portability database includes a LRN (Local Routing Number) which can be used for orientation to the IMS network.
Step 230, the circuit domain sends an IAM message to the MGCF, wherein the called party number is the LRN, and the original called party number is stored in an ISUP Ported Gap information element.
Step 240, upon the MGCF finds that this is number portability, the MGCF extracts the original called party number and sends an Invite message to the I/S-CSCF.
Step 250, the S-CSCF triggers a service to the VCC AS according to an iFC (initial Filter Criteria).
According to the above description, the call termination of the current CS domain can be anchored only when an intelligent network service or number portability is deployed, however, intelligent network service or number portability have not been deployed in many of the current circuit switch network. In this case, the call termination of the CS domain in a VCC service cannot be anchored.